Electrical energy can be conserved by selecting and limiting the hours during which it is consumed. Street lamps, for example, are commonly turned on at sunset and off at sunrise by a photoelectric cell. More widespread programming of household hot water heaters during hours off the peak of consumption would allow electrical utilities to operate more efficiently. On the one hand, however, photoelectrically controlled street lamps burn until sunrise, whereas at least half of them on a given stretch of road might be safely turned off at two o'clock in the morning. On the other hand the off-peak electrical power hours do not start until late in the evening, well after sunset and continue until sunrise. Supply of power to the heater might be limited to certain hours by a clock, but clocks are known to drift in accuracy over periods of months, and may be radically detimed by power outages.
It is therefore one object of the present invention to provide an energy conserving power control circuit which eliminates the errors of asynchronous clock operation. A further object is to eliminate malfunction of a photoelectric device by accidental or intentional covering of its photosensitive surface. A still further object is to program use of electrical power in the most economical hours.